The invention relates to curing. The invention finds particular application in the curing of ink, in particular the curing of radiation-curable inks. Preferred examples of the invention relate to UV curing. Particularly preferred examples of the invention relate to the curing of ink jet inks, in particular UV curable ink jet ink. Other aspects of the invention relate to ink compositions.
While embodiments of the invention described herein relate to ink, which is used to print a graphic image, the invention has general applicability to other curable fluids, and need not relate to printed graphical images, or even to printed images. For example, the fluid could comprise a pcb etch resist, plastic electronic material or other material. Where reference is made to “ink” herein, it is to be understood that, preferably, that term is to include reference to other fluids, where appropriate.
Furthermore, while in examples described herein the fluid is applied to a substrate by printing, other methods could be used.
The use of curable inks in printing is well known. Curable ink is preferably to be understood to include ink that solidifies by reaction, in particular for example polymerisation and/or crosslinking. For many curable inks, the ink (for example liquid ink) is solidified by exposing the ink to radiation. Of particular interest is UV curing ink.
In the use of UV curable inks, the ink is deposited on a substrate using a suitable method. Exposing the ink on the substrate to UV light effects cure of the ink. In some examples, the exposure of the ink to UV light initiates a chemical reaction that turns the liquid ink into a solid. In other examples, curing can be effected using other curing radiation, for example gamma radiation. Radiation curable inks may be cured using an electron beam, for example from an electron gun. Some inks can be cured simply by applying heat, for example employing an IR source. However, the heat input required to achieve a temperature for rapid cure is often too high for this to be an attractive method. The ink may comprise aqueous UV ink.
One of the main problems in designing printers to print using curable ink is the provision of a suitable radiation source to effect the curing. For UV curing, the most widely used technology is a mercury discharge lamp; examples of such lamps are mercury lamps produced by Primarc UV Technology (NJ, USA). However, such lamps have a number of disadvantages.
Firstly, little of the electrical energy consumed by the lamp is converted into UV energy. Typically only 10 to 15% of the input power effects emission in the desired wavelengths of 250 to 390 nm. The remainder is either emitted at other (mainly longer) wavelengths, or as heat. This heat must be taken away from the lamp by conduction or convection. This waste heat can cause problems because it can effect heating of the substrate to which the ink is being applied, and also because the lamp requires active cooling which is expensive, especially if the UV lamp is mounted on a moving carriage on the printing apparatus.
Secondly, the UV output obtained from the lamp is highly sensitive to the operating temperature of the lamp. It is difficult to control the operating temperature accurately, and hence to ensure that the emitted UV is constant. A more important problem is the speed of response of the lamp. Starting from cold, it can take half a minute or more to warm the lamp up to full operating temperature, during which time the UV output is rising to its rated value. This can be a particular problem for printers where the UV is needed intermittently during the scanning of the printhead assembly over the substrate. Keeping the UV lamp at full power all the time would be wasteful, and would also cause further problems associated with stray UV radiation from the lamp which could have safety implications and might lead to unwanted curing of the ink in the printheads themselves.
Typically the bulb of the UV lamp is held in “standby mode” when it is not in use, at a reduced power level of perhaps 20% of rated input power. This keeps the bulb warm, but there is still a substantial amount of waste heat generated, which must be removed. In practice, because the lamp cannot be switched on and off quickly from cold, a mechanical shutter is often used to reduce the amount of stray UV emitted when the lamp is in standby mode.
The spectrum of UV emission from mercury arc lamps is in a number of very sharp peaks (emission lines), which are spread over a range of wavelengths. When considering the target material, for example a UV curing ink, it can be seen that it is very difficult to match the sensitivity of the ink to a set of very narrow emission lines which are spread over a range of frequencies. In practice, a combination of several initiators are used in the inks, which are designed to absorb over a broad range of frequencies so that they can make use of most of the UV output of the lamp. This tends to make the inks more sensitive to stray light (for example sunlight) which is often broad band. This can increase the likelihood that the ink is cured by stray light before it is deposited on the substrate.
Also, in many cases, the intensity of UV output along the length of a mercury discharge lamp is uneven. This appears to be an unavoidable property of the lamps in practice, and can result in variations in the appearance of the final ink film.
Furthermore, the lamps (together with their reflectors, cooling and shuttering) are bulky and heavy. This is particularly a disadvantage when the lamps are mounted on a moving carriage.